There are many processes in the petrochemical industry that use catalysts and adsorbents. The catalysts and adsorbents are frequently transferred between operational units and regeneration units, and often there is a continuous flow of the catalyst and/or adsorbent through the system comprising the operational unit and the regeneration unit.
One common system that requires the continuous flow of solids is a transport reactor. In hydrocarbon processing, transport reactors are commonly used. In a transport reactor the catalyst bed moves through the reactor where the catalyst contacts the reactants. This is different from fixed bed reactors where the catalyst is held in place by screens or other devices, or ebullated bed reactors where the catalyst particles circulate within the reactor, but are not carried out of the reactor. In a transport reactor, the catalyst is carried through the reactor by the fluid reactants passing through the reactor. Although the general direction for a transport reactor is in an upward direction, it can also be downward, horizontal, or at some angle in between horizontal and vertical. In the case where the reactor is vertical and the transport is in the upward direction, the reactor is called a riser reactor, where the catalyst is introduced in the bottom of the reactor and is carried up through the reactor.
Riser reactors are commonly used in hydrocarbon processing. A fluid hydrocarbon reactant contacts a solid catalyst carried along by a fluid, where the catalyst and fluid are introduced at the bottom of the reactor and the fluid and catalyst rise up through the reactor in a fluidized state during which the process reaction takes place. Upon exiting the riser reactor, the fluid and catalyst enter a separation zone where the catalyst disengages from the fluid and settles by gravity to the bottom of the separation zone. The catalyst is then withdrawn and sent to a regeneration unit, before recirculation to the riser reactor.
Several methods are used for controlling the introduction of solid catalysts to the bottom of a riser reactor, or for any process unit where a solid is introduced and carried through the process unit. The means for control include slide valves, lock hoppers, screw conveyors and L-valves, to name a few. The L-valve is a specific type of a non-mechanical valve. These valves have no moving parts and control the flow of solids through the introduction of a fluid to carry the solids along. In an L-valve, solids are fed by gravity to a downcomer, or a vertical pipe. The downcomer intersects a horizontal pipe, or exit arm, giving the appearance similar to the letter “L”. A motive fluid inlet located at the junction opposite the exit arm, or intersecting the vertical leg proximate to the “L” junction provides the energy to carry the solids out the exit arm. Control of the L-valve is through the control of the flow of a motive fluid to carry the solids out the exit arm. The solids flow includes fluid transported with the solids down through the vertical pipe, where the fluid in the horizontal pipe provides a motive force to facilitate all of the fluid carrying solids out the second arm, or exit of the L-valve. Generally, the solids flow rate can be controlled by adjusting the rate at which fluid is introduced at the junction. However, there are control problems due to flow instabilities for certain flow regimes, and notably when the fluid is a liquid.
Accordingly, there is a need for improved apparatuses to reliably feed solids at a controlled and reproducible rate while reducing the attrition of catalysts and adsorbents in systems with continuous solids circulation.